1. Field of the Invention
The present invention relates to hearing aids. The invention further relates to methods of signal processing within a hearing aid. The invention more particularly relates to hearing aids with multiple input transducers and to methods of signal processing in hearing aids with multiple input transducers. The invention, still more particularly, relates to hearing aids with multiple input transducers adapted to provide an adjustable directivity pattern.
The invention, yet more specifically, relates to an input processor for processing of input transducer signals in a hearing aid, wherein input signals are processed in a directional controller and wherein feedback-compensating signals are combined with signals derived from the input signals.
2. The Prior Art
WO-A-01/01731 shows the use of an adjustable directional microphone system for a hearing aid. The system processes inputs from two microphones according to acoustical time delays to achieve a directional sensitivity pattern. The processor may also compensate the suppression of low frequency signals inherent to directional processing, an action sometimes referred to as equalizing or low frequency boosting. Directional processing is typically used to suppress environmental noise in situations where the hearing aid user wants to suppress sounds impinging from directions other than that towards a conversational partner. Equalizing generally boosts the low frequency signals, whether they are regarded as signals of interest or noise, and therefore may cause problems on its own.
WO-A-02/085066 shows a directional system, which is adaptively controlled. The directional controller may be implemented in a multi-channel version, i.e. with delay processors in respective frequency bands.
EP-A-1191814 shows a system for alleviating a disturbance known as acoustical feedback. Acoustical feedback refers to the incidence at the microphone of an acoustic signal generated by the output transducer. The feedback signal is likely to be picked up by the microphone and amplified by the hearing aid processor to give rise to an output that will again loop back to the microphone. If the gain exceeds the attenuation factors in the loop, an unstable situation will arise. Feedback may give rise to distortion of the signal, even at gain settings below the instability limit. EP-A-1191814 describes an adaptive feedback compensation (FBC) system, wherein a feedback-compensating signal is subtracted from the output of the microphone system in order to produce a combination signal, which is then fed to the main signal processor.
The feedback compensation signal is generated in a feedback signal predictor that monitors the output signal from the main signal processor, i.e. the signal fed to the output transducer of the hearing aid, and the input signal to the main signal processor. By correlating these signals, the feedback predictor can work out an estimate of the feedback path from the processor output and back to the processor input. The feedback path thus estimated generally incorporates the output transducer, the acoustic path back to the microphone, the microphone, and any preamplifiers. The feedback path is characterized by a transfer function. The feedback signal predictor—often referred to as a feedback signal estimator—comprises a filter that is adaptively controlled according to the correlation between said main signal processor output signal and the combination signal. The prevalence of high correlation is presumed to be due to acoustic feedback, and the feedback signal predictor in this way generates an estimate of the feedback path and produces a cancellation signal, which is then subtracted from the signal outputted by the microphone system. The feedback compensation feature allows the main signal processor to operate at a higher gain than otherwise possible.
WO-A-01/10169 shows a hearing aid with a controllable directional characteristic and with adaptive matching of input transducers. A controllable filter is inserted in at least one of two microphone channels for the purpose of equalizing the microphone output signals in gain and phase characteristics, which is important for the proper functioning of the directional systems.
WO-A-99/26453 shows a feedback compensation system for a hearing aid with two microphones and directional processing, wherein each microphone signal is independently feedback compensated before processing in a directional controller. Independently compensating each microphone signal before directional processing requires extensive processing and carries a risk that an imperfect compensation of the feedback signals will result in a residual feedback signal component, which may interfere with the function of the directional controller.
Generally, a feedback estimator estimates the transfer function in a part of the feedback loop extending from the signal processor output to the signal processor input. This part of the feedback loop mainly includes the output transducer, the acoustic path from output port to input port, the input transducer and circuitry associated with the input transducer.
The acoustic part of the feedback path may, according to a simple model, be regarded as a frequency dependent, attenuation and delay function. As the part of the feedback loop to be estimated actually includes on top of the acoustic path the output transducer, the input transducer and input circuitry, the complexity of this part of the feedback path may be considerable, especially in case of advanced hearing aids, and more sophisticated models may be appropriate to adequately mimic the feedback path.
Adaptive systems are examples of non-linear devices, or devices that can only be regarded as linear in short time segments. Non-linear devices present in an advanced input signal processor may include e.g. directional controllers, microphone matching circuits, preamplifiers and noise processors, and possible even adaptive versions of these systems.